In the continuing effort to improve performance of transistors and integrated circuits (ICs) in which they are used, semiconductor device designers strive to increase the switching speed of the transistors as manufacturing technology advances. One aspect of this effort includes shrinking the dimensions of transistor features. As physical dimensions of the transistor decrease, some undesirable effects may become non-negligible.
One structural feature of some MOS transistors is a source/drain extension. The source/drain extension is a relatively shallow portion of the source/drain region adjacent the gate electrode of the transistor. One purpose served by the source/drain extension is to reduce the electrical field gradient at the transition from the source or drain to the channel region of the transistor. Reduction of the gradient reduces hot carrier effects that may result in reduced lifetime or performance stability of the transistor. Design objectives with respect to the source/drain extensions include low sheet resistance and shallow junction depth.